Don't know if this helps you out, but I found these extension definitions in GIS's knowledgebase.
http://gis.sfsu.edu/helpdesk/arcmap/datalayers.htm--------------------------
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What are auxiliary(*.aux) and pyramid (*.rrd) files and are they necessary?
The .aux extension indicates an auxiliary file used to store information not normally supported by the particular raster format. The TIFF file format, for example, does not store image statistics, so statistics generated by ArcGIS are stored in an .aux file with the same prefix name as the .tif file
The .rrd extension indicates an pyramid file used to speed raster display. When a raster has to be represented in a series of reduced/increased resolutions, a pyramid is built for that particular raster. A pyramid is a series of reduced resolution representations of the dataset, mainly used to improve the display performance of rasters when one is not working with the pixel information at full resolution. It conatins a number of layers, each resampled at a more generalized level. Thus each level of the pyramid is a resampled representation of the raster at a coarser spatial resolution.
The data is viewable without this and the auxiliary file, however loading times will be slower.
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I defined the projection of an image but it still doesn't have spatial reference information when added to an ArcMap dataframe. Why?
There a couple reasons why the spatial reference of an image defined in ArcCatalog or ArcToolbox isn't recognized in an ArcMap dataframe. This is generally related to one of two things, the world file or pyramid layers.
World files are small text files that contain the size and position (georeference) information for an image. A typical world file might look like:
20.154 <the dimension of a pixel in project units in the x direction>
0.000 <rotation term for row>
0.000 <rotation term for column>
-20.154 <the dimension of a pixel in project units in the y direction>
424178 <the x coordinate of the center of pixel 1,1 (upper-left pixel)>
4313415 <the y coordinate of the center of pixel 1,1 (upper-left pixel)>
World file formats have different extensions for different image types. When an image is read (using menu File | Open or File | Insert File), ArcMap looks for a matching world file using the following extensions.
*.tfw - world file for tif image
*.bpw - world file for bmp images
*.jgw - world file for jpg
If no world file exists, the image is inserted so that the upper left corner is in the center of the screen and the pixel size is set to one. Despite defining a projection, the lack of a world file (or a misnamed world file) will cause the image to not display correctly in ArcMap.
It should also be noted that GeoTiff files are Tif images with georeference information embedded in them. ArcMap will read GeoTiff files. GeoTiff files (such as some USGS DRGs) strictly do not require a world file because they already have that information embedded in the file. In this case if a world file exists, the information in the world file will override the embedded information.
When a raster has to be represented in a series of reduced/increased resolutions, a pyramid is built for that particular raster. A pyramid is a series of reduced resolution representations of the dataset, mainly used to improve the display performance of rasters when one is not working with the pixel information at full resolution. When pyramids are created the spatial reference of the dataset defined at the time of pyramid creation is hard-coded into the pyramid layers.
If pyramid layers are built for a dataset before defining its projection, setting the spatial reference will have no affect in ArcMap becasue the pyramid layers (with the hard-coded projection, e.g. unknown or wrong) are overriding the defined projection. To correct for this, delete the .rrd and .aux files of the dataset using Windows Explorer, redefine the spatial reference information in ArCatalog or ArcToolbox to the correct projection, and then build pyramids
Note: The data is still viewable without the pyramid and the auxiliary file, however loading times will be slower.
